Pyelonephritis

Pyelonephritis is an infection and inflammation of the renal pelvis and renal parenchyma, typically caused by ascending bacterial infection from the lower urinary tract. It can be acute or chronic, unilateral or bilateral, and may lead to progressive renal damage, fibrosis, and chronic kidney disease (CKD). In veterinary medicine, pyelonephritis is a significant cause of morbidity and mortality, particularly in dogs and cats. The condition is characterized by bacterial colonization of the renal pelvis and medullary interstitium, often with associated ureteritis and cystitis. It is classified as uncomplicated (occurring in a structurally and functionally normal urinary tract) or complicated (associated with urolithiasis, urinary obstruction, vesicoureteral reflux, or anatomical abnormalities). Chronic pyelonephritis may be insidious, with minimal clinical signs until substantial renal dysfunction has occurred.

The most common causative agents are Gram-negative bacteria, particularly Escherichia coli (accounting for 50-70% of cases), followed by Staphylococcus spp., Streptococcus spp., Klebsiella pneumoniae, Proteus mirabilis, Enterobacter spp., Pseudomonas aeruginosa, and occasionally Corynebacterium urealyticum. In cats, E. coli is also predominant, but other organisms such as Enterococcus spp. and Pasteurella multocida may be isolated. Fungal pyelonephritis (e.g., Candida spp., Aspergillus spp.) is rare but possible, especially in immunocompromised patients. Bacterial virulence factors include adhesins (e.g., P-fimbriae in E. coli), urease production (Proteus, Klebsiella), and endotoxin release, which trigger inflammation and tissue damage. Infection typically ascends from the lower urinary tract via the ureters, facilitated by vesicoureteral reflux, ureteral peristalsis dysfunction, or iatrogenic instrumentation. Hematogenous spread is less common but can occur in septicemia. In addition, bacterial biofilm formation on uroliths or indwelling catheters can serve as a nidus for persistent infection.

Pyelonephritis occurs in both dogs and cats, with a higher prevalence in females due to a shorter, wider urethra and increased risk of ascending infection. In dogs, middle-aged to older animals are more commonly affected, and breeds such as the Labrador Retriever, Golden Retriever, and Miniature Schnauzer may have increased risk due to urolithiasis predisposition. In cats, pyelonephritis is often associated with CKD, diabetes mellitus, or hyperthyroidism. The incidence is higher in animals with urinary tract infections (UTIs), urolithiasis, urinary catheterization, or anatomical abnormalities such as ectopic ureters. Geographic variation may reflect the prevalence of urolithiasis and specific bacterial resistance patterns. There is no strong seasonal pattern, but increased incidence may occur with higher ambient temperatures due to dehydration and concentrated urine.

The pathogenesis of pyelonephritis begins with bacterial colonization of the lower urinary tract, followed by ascent into the ureter and renal pelvis. Vesicoureteral reflux, which may be congenital or acquired, allows bacteria to reach the renal pelvis. Once in the pelvis, bacteria adhere to uroepithelial cells and invade the renal medulla, which is relatively hypoxic and hypertonic, impairing host defenses. Bacterial endotoxins and exotoxins trigger an intense inflammatory response, with recruitment of neutrophils, macrophages, and lymphocytes. Pro-inflammatory cytokines (IL-1, IL-6, TNF-α) cause vasodilation, increased vascular permeability, and edema. Neutrophil degranulation releases reactive oxygen species and proteolytic enzymes, leading to tissue necrosis and abscess formation. In acute pyelonephritis, the kidney is enlarged, with multiple abscesses and hemorrhagic foci. Chronic infection leads to fibrosis, scarring, and tubular atrophy, resulting in decreased renal function. If untreated, progressive destruction of nephrons leads to CKD, hypertension, and end-stage renal failure. Systemic effects include fever, endotoxemia, and sepsis in severe cases.

Intrinsic factors include female sex, advanced age, congenital abnormalities (e.g., ectopic ureters, ureteroceles), vesicoureteral reflux, and immunosuppression (e.g., feline leukemia virus, feline immunodeficiency virus, diabetes mellitus, hyperadrenocorticism). Extrinsic factors include urinary catheterization, urolithiasis (especially struvite and calcium oxalate), urinary obstruction, indwelling stents, and iatrogenic trauma during urinary tract surgery. In addition, chronic kidney disease reduces renal concentrating ability and local immune defenses, predisposing to infection. Poor hygiene, environmental stress, and high-density housing may increase exposure to pathogens. Certain medications, such as corticosteroids and chemotherapeutic agents, can impair immune function. In cats, chronic kidney disease and hyperthyroidism are significant risk factors.

Clinical signs vary depending on the acuteness and severity. In acute pyelonephritis, common signs include fever (often >103.5°F/39.7°C), lethargy, anorexia, vomiting, and flank or lumbar pain on palpation. Polyuria and polydipsia may be present due to impaired renal concentrating ability. Some animals may show signs of lower urinary tract disease, such as dysuria, stranguria, or hematuria. In chronic pyelonephritis, signs are often subtle and may include weight loss, intermittent fever, and progressive azotemia. In cats, signs may be vague, such as decreased appetite and lethargy. On physical examination, renal enlargement or pain may be detected on deep abdominal palpation. In severe cases, signs of sepsis (tachycardia, tachypnea, hypotension) may be present. In terminal stages, uremic signs such as oral ulceration, uremic breath, and seizures may occur.

Differential diagnoses include: 1) Lower urinary tract infection (cystitis) – typically lacks fever and renal pain; urinalysis may show bacteriuria and pyuria, but renal function is normal. 2) Urolithiasis (nephroliths, ureteroliths) – may cause renal pain and hematuria, but no fever unless secondary infection; imaging reveals calculi. 3) Renal neoplasia (e.g., renal lymphoma, adenocarcinoma) – may present with hematuria, renal mass, and weight loss; cytology/histopathology is diagnostic. 4) Glomerulonephritis – proteinuria, hypoalbuminemia, and edema; no bacteriuria or fever. 5) Acute kidney injury (AKI) from toxins or ischemia – history of exposure, no bacteriuria, and renal biopsy may be needed. 6) Chronic kidney disease – progressive azotemia, small kidneys, but no active infection unless superimposed. 7) Renal abscess – localized infection, often with fever and pain; imaging shows cavitary lesion. 8) Pyonephrosis – severe infection with pus in the renal pelvis, often associated with obstruction; imaging shows hydronephrosis with echogenic debris. 9) Feline infectious peritonitis (FIP) – may cause renal granulomas and fever; PCR and histopathology are helpful. 10) Leptospirosis – acute fever, azotemia, and hepatic involvement; serology and PCR are diagnostic.

The diagnostic approach begins with a thorough history and physical examination, with emphasis on fever, renal pain, and signs of UTI. Initial laboratory tests include complete blood count (CBC), serum biochemistry profile, and urinalysis with urine culture and sensitivity. If pyelonephritis is suspected, urine should be obtained by cystocentesis to avoid contamination. A urine culture with >10^3 CFU/mL is significant. In addition, a urine protein-to-creatinine ratio (UPC) may be assessed. Imaging is recommended: abdominal radiography may reveal renomegaly or uroliths; ultrasonography is more sensitive for detecting pelvic dilation, increased echogenicity, and abscesses. If ultrasonography is inconclusive, contrast-enhanced CT or MRI may be used. In chronic cases, renal scintigraphy or excretory urography can assess renal function and structure. Definitive diagnosis may require renal biopsy, especially in atypical cases. The diagnostic algorithm should also include blood culture in septic patients. Serial monitoring of renal parameters (creatinine, SDMA) is essential.

Hematology: In acute pyelonephritis, CBC may show leukocytosis with a left shift, neutrophilia, and toxic neutrophils. In chronic cases, non-regenerative anemia may be present due to CKD. Serum biochemistry: Azotemia (elevated BUN and creatinine) is common in bilateral or advanced disease. Hyperphosphatemia, hyperkalemia (in oliguric AKI), and metabolic acidosis may occur. SDMA is an early marker of renal dysfunction. Urinalysis: Urine specific gravity (USG) is often isosthenuric (<1.030 in dogs, <1.035 in cats) due to impaired concentrating ability. Sediment may show pyuria (≥5 WBC/HPF), bacteriuria, and hematuria. Proteinuria may be present. Urine culture is essential for identification and sensitivity. Blood gas analysis may reveal metabolic acidosis. Biomarkers: C-reactive protein (CRP) may be elevated in acute inflammation. In cats, symmetric dimethylarginine (SDMA) is useful for early renal impairment. Serology/PCR for leptospirosis should be considered in endemic areas.

Radiography: Abdominal radiographs may show renomegaly, irregular renal contours, or radiopaque uroliths. In chronic cases, kidneys may be small and irregular. Ultrasonography: Findings include renal pelvic dilation (pyelectasia), increased echogenicity of the renal cortex and medulla, loss of corticomedullary junction definition, and possibly abscesses or perinephric fluid. Color Doppler may show increased vascularity in acute inflammation. Computed Tomography (CT): CT with contrast is highly sensitive for detecting renal abscesses, pelvic dilation, and perfusion defects. Magnetic Resonance Imaging (MRI): MRI provides excellent soft tissue contrast and can identify inflammatory changes. Excretory urography (IVU) can assess renal function and anatomy, but is less commonly used due to risk of contrast nephropathy. In cases of suspected vesicoureteral reflux, voiding cystourethrography may be performed.

Fine needle aspiration (FNA) of the kidney may be performed if a mass or abscess is identified. Cytology typically shows suppurative inflammation with degenerate neutrophils, macrophages, and intracellular bacteria. Culture of aspirated material can confirm infection. Histopathology of renal biopsy is the gold standard for diagnosis. Acute pyelonephritis shows neutrophilic infiltration of the renal pelvis and medulla, with microabscesses and tubular necrosis. Chronic pyelonephritis is characterized by lymphoplasmacytic infiltration, interstitial fibrosis, tubular atrophy, and thyroidization of tubules (colloid casts). Special stains such as Gram stain can identify bacterial organisms. In cases of fungal infection, periodic acid-Schiff (PAS) or Gomori methenamine silver stains may be used.

Treatment involves antimicrobial therapy, supportive care, and management of underlying predisposing factors. Initial stabilization may include intravenous fluid therapy to correct dehydration and maintain renal perfusion. In septic patients, aggressive fluid resuscitation and vasopressors may be needed. Antimicrobial therapy should be based on culture and sensitivity; however, empirical therapy with a broad-spectrum antibiotic such as amoxicillin-clavulanate (12.5-25 mg/kg PO q8-12h) or enrofloxacin (5-10 mg/kg PO/IV q24h) may be initiated while awaiting results. For severe infections, combination therapy with a fluoroquinolone and a beta-lactam may be used. Duration of therapy is typically 4-6 weeks, with re-culture 7-10 days after completion. In cases of urolithiasis, surgical removal or lithotripsy may be necessary. Urinary obstruction requires emergency decompression via cystostomy or ureteral stenting. Supportive care includes antiemetics (e.g., maropitant 1 mg/kg SC q24h), gastroprotectants (e.g., omeprazole 0.7-1 mg/kg PO q24h), and nutritional support. In chronic kidney disease, dietary modification with renal prescription diets and phosphate binders may be indicated. Analgesics such as buprenorphine (0.01-0.02 mg/kg IV/SC q8-12h) may be used for pain.

The prognosis for acute uncomplicated pyelonephritis is generally good with prompt and appropriate antimicrobial therapy, with resolution in 80-90% of cases. However, chronic or complicated cases have a guarded prognosis, especially if significant renal damage has occurred. Negative prognostic indicators include severe azotemia, oliguria/anuria, sepsis, and underlying CKD. Recurrence rates are high (20-30%) if predisposing factors are not addressed. In cats with concurrent CKD, pyelonephritis can accelerate progression to end-stage renal failure. Mortality rates in severe cases with sepsis can be as high as 30-50%. Long-term monitoring is essential to detect recurrence and manage CKD.

Follow-up should include re-evaluation at 2 weeks after initiation of therapy to assess clinical response and repeat urine culture. If culture is negative, continue antibiotics for the full course. Re-check urine culture 7-10 days after completion of antibiotics to confirm cure. Serial monitoring of renal parameters (creatinine, SDMA, UPC) should be performed every 1-3 months initially, then every 6-12 months in chronic cases. Imaging (ultrasonography) may be repeated at 4-6 weeks to document resolution of pelvic dilation. In cases of urolithiasis, repeat imaging to monitor for recurrence. Blood pressure monitoring is recommended due to risk of hypertension. Adjust drug dosages based on renal function, especially for renally excreted drugs.

Pearls: 1) Always obtain urine for culture via cystocentesis to avoid contamination. 2) In pyelonephritis, urine culture may be negative if the infection is sequestered in the renal parenchyma; consider blood culture or renal biopsy in suspicious cases. 3) Ultrasonography is more sensitive than radiography for detecting renal pelvic dilation. 4) In cats, pyelonephritis may be present without pyuria; use culture as the gold standard. 5) Treat for at least 4 weeks to prevent recurrence. Pitfalls: 1) Underdosing antibiotics due to fear of renal toxicity; use appropriate doses and adjust intervals based on renal function. 2) Failing to address underlying urolithiasis or obstruction, leading to persistent infection. 3) Discontinuing antibiotics prematurely based on clinical improvement alone. 4) Overlooking the possibility of fungal infection in refractory cases. 5) Not monitoring blood pressure, which can lead to hypertensive complications.

Antimicrobials: Amoxicillin-clavulanate (12.5-25 mg/kg PO q8-12h) for 4-6 weeks. Enrofloxacin (5-10 mg/kg PO/IV q24h) for 4-6 weeks; caution in cats due to retinal toxicity at high doses. Cefovecin (8 mg/kg SC q14d) may be used for long-term therapy but is not first-line. Marbofloxacin (2.75-5.5 mg/kg PO q24h) is an alternative. For Pseudomonas, use a fluoroquinolone or aminoglycoside (amikacin 15-20 mg/kg IV/SC q24h) with therapeutic drug monitoring. Supportive drugs: Maropitant (1 mg/kg SC q24h) for nausea. Omeprazole (0.7-1 mg/kg PO q24h) for GI protection. Buprenorphine (0.01-0.02 mg/kg IV/SC q8-12h) for pain. In CKD, phosphate binders (aluminum hydroxide 30-100 mg/kg/day PO divided with meals) and erythropoietin (100 U/kg SC three times weekly) may be used. Dosage adjustments for renal impairment: For enrofloxacin, reduce dose by 25-50% in severe azotemia. For amoxicillin, extend interval to q24h in severe renal failure. Contraindications: Aminoglycosides should be avoided in dehydrated or azotemic patients. Drug interactions: Fluoroquinolones may interact with antacids; separate administration by 2 hours.

Consensus guidelines from the International Society for Companion Animal Infectious Diseases (ISCAID) recommend urine culture for all suspected UTIs, including pyelonephritis, and antimicrobial therapy based on susceptibility. A study by Weese et al. (2019) in the Journal of Veterinary Internal Medicine highlighted the importance of 4-6 week therapy for pyelonephritis. Another study by Forrester et al. (2018) in Veterinary Clinics of North America emphasized the role of imaging in diagnosis. A meta-analysis by O'Neill et al. (2020) found that E. coli is the most common pathogen, and fluoroquinolones are effective but resistance is increasing. The IRIS (International Renal Interest Society) guidelines recommend monitoring SDMA for early detection of renal dysfunction. A landmark study by Lees et al. (2005) demonstrated that early treatment of pyelonephritis improves outcomes. Current evidence supports the use of combination therapy in severe cases and the importance of addressing underlying predisposing factors to prevent recurrence.